In recent years, OCT that forms images of the surface morphology and internal morphology of an object by using a light beam from a laser light source or the like has attracted attention. Unlike an X-ray CT apparatus, OCT is noninvasive to human bodies, and is therefore expected to be utilized in the medical field and biological field. For example, in the ophthalmology, apparatuses that form images of a fundus and a cornea are in a practical stage.
Japanese Unexamined Patent Application Publication No. Hei 11-325849 discloses a device to which OCT is applied. This device has such a configuration that: a measuring arm scans an object by a rotary deflection mirror (a Galvano mirror); a reference arm is provided with a reference mirror; and an interferometer is mounted at the outlet to analyze, by a spectrometer, the intensity of an interference light of light fluxes from the measurement arm and the reference arm. Moreover, the reference arm is configured to gradually change the light flux phase of the reference light by discontinuous values.
The device disclosed in Japanese Unexamined Patent Application Publication No. Hei 11-325849 uses a technique of so-called “Fourier Domain OCT.” That is to say, the device irradiates a low coherence light beam to an object, superposes the reflected light and the reference light to generate an interference light, and acquires the spectral intensity distribution of the interference light to execute Fourier transform, thereby imaging the morphology in the depth direction (the z-direction) of the object. The technique of this type is also called Spectral Domain.
Furthermore, the device described in Japanese Unexamined Patent Application Publication No. Hei 11-325849 is provided with a Galvano mirror that scans with a light beam (a signal light), and is thereby configured to form an image of a desired measurement target region of the object. This device is configured to scan with the light beam only in one direction (the x-direction) orthogonal to the z-direction. An image formed by this device is a two-dimensional tomographic image in the depth direction (the z-direction) along the scanning direction (the x-direction) of the light beam.
Japanese Unexamined Patent Application Publication No. 2002-139421 discloses a technique of scanning with a signal light in the horizontal direction (x-direction) and the vertical direction (y-direction) to form multiple two-dimensional tomographic images in the horizontal direction, and acquiring and imaging three-dimensional tomographic information of a measured range based on the tomographic images. As the three-dimensional imaging, for example, a method of arranging and displaying multiple tomographic images in the vertical direction (referred to as stack data or the like), and a method of executing a rendering process on volume data (voxel data) based on the stack data to form a three-dimensional image are considered.
Japanese Unexamined Patent Application Publication No. 2007-24677 and Japanese Unexamined Patent Application Publication No. 2006-153838 disclose other types of OCT devices. Japanese Unexamined Patent Application Publication No. 2007-24677 describes an OCT device that images the morphology of an object by scanning (sweeping) the wavelength of light that is irradiated to an object, acquiring the spectral intensity distribution by detecting interference light obtained by superposing the reflected lights of the light of the respective wavelengths on the reference light, and executing Fourier transform. Such an OCT device is called a Swept Source type or the like. The Swept Source type is a kind of the Fourier Domain type.
Further, Japanese Unexamined Patent Application Publication No. 2006-153838 describes an OCT device that irradiates a light having a predetermined beam diameter to an object and analyzes the components of an interference light obtained by superposing the reflected light and the reference light, thereby forming an image of the object in a cross-section orthogonal to the travelling direction of the light. Such an OCT device is called a full-field type, en-face type or the like.
Japanese Unexamined Patent Application Publication No. 2008-73099 discloses a configuration obtained by applying OCT in the ophthalmic field. It should be noted that before the use of OCT, a retinal camera and a slit lamp microscope have been widely used as devices for observing a subject eye (refer to, for example, Japanese Unexamined Patent Application Publication No. Hei 9-276232 and Japanese Unexamined Patent Application Publication No. 2008-259544). The retinal camera is a device that photographs the fundus oculi by projecting illumination light onto the eye and receiving the reflected light from the fundus oculi. The slit lamp microscope is a device that obtains an image of the cross-section of the cornea by cutting off the light section of the cornea using slit light.
A device using OCT is advantageous compared to a retinal camera etc. with respect to the fact that it is capable of acquiring high-definition images, and is also capable of acquiring tomographic images and three-dimensional images.
In this manner, the device using OCT may be applied to the observation of various sites of the subject eye, and because high-resolution images may be obtained, it is applied to the diagnosis of various ophthalmologic diseases.
What is important not only in ophthalmology but to the general medical field is to appropriately image the region of interest. Particularly in ophthalmology, the subject eye, which is the photographing subject, has a very fine structure and accompanies eye movement; therefore, it is not easy to accurately designate the scanning location of a signal light.
Designation of the scanning location is conducted with reference to a real-time moving image (near-infrared fundus image) that uses a near-infrared light. For example, in the OCT measurement of the fundus, there is a method of scanning the fundus with a laser beam in the near-infrared region and imaging a two-dimensional distribution of the reflection intensity from each point (SLO, Scanning Laser Ophthalmoscope) and a method of photographing the fundus lit with a near-infrared light. According to the former, high resolution is achieved by a confocal optical system; however, there is a demerit in that the structure is complicated and expensive. Meanwhile, regarding the latter, it may be achieved by a simple optical system as in conventional retinal cameras; however, because confocality is not used, reflected lights (background components) from optical elements and/or from various depth locations of fundus get mixed, preventing high-resolution such as SLO from being obtained. Accordingly, techniques such as for adjusting the contrast of images, gamma adjustment, etc., are employed (for example, refer to Japanese Unexamined Patent Application Publication No. 2004-41371, Japanese Unexamined Patent Application Publication No. 2004-313455, and Japanese Unexamined Patent Application Publication No. Hei 11-238129).